Although three state multiplexing has been around for some time now,
there are still a few mysteries that havent gotten around too well yet.
There are two problems in particular that i would like to discuss,
with the first being rather simple but the second even Wikipedia
didnt get right.
One is the total number of LEDs that can be scanned with a given
number of control lines. This is easily solved however by examining
a few examples and realizing that it's the simple permutations forumla:
where the exclamation point represents factorial and is equal to:
so for n=3, n!=6.
What P(n) tells us is the total number of LEDs that can run off of
a given number of control lines, n, so we know what to expect.
For example, say we only have 3 control lines...
so, we can control 6 LEDs with 3 control lines.
Also of some interest is that the formula, knowing the number
of LEDs for a given n, when we want to increase the number of
control lines by 1 is even simpler:
or stated another way, if n=3 we know that this will control 6
LEDs, so if we increase n by 1 (to n=4 for four control lines)
the number of LEDs would increase by 6, because 2*3=6, so
we would end up being able to control a total of 12 LEDs
The second problem is a lot less commonly known. Wikipedia
as of 12/11/2008 states that each LED is driven one at a time,
and although this CAN be the way to do it, it's not the ONLY way,
and in fact this takes a much higher scan frequency and also may
reduce the duty cycle too low to be successfully used for some
number of LEDs or LED segments. There is a limit that is reached
once we get to a larger number of LEDs where if you turn one on
at a time you reduce the duty cycle so low that it would take a
current that is much above the peak rating of the LED to drive it.
Even a 5 to 1 peak rating isnt good enough in some cases.
This means an alternate way of scanning has to be incorporated
in order to both scan the LEDs correctly and also to keep the duty
cycle high enough so that a reasonable peak current can be
employed to make sure the LEDs shine bright enough for a typical
If the LEDs are divided up into rows and columns similar to when
standard multiplexing is used but instead using three state
multiplexing the LED duty cycle can be kept higher and still
able to drive more LEDs than with standard multiplexing.
I had worked out a scheme for 100 LEDs but i am a loss as to
where it is on the web now, especially since AOL has graciously
discontinued allowing members to post web pages in Hometown
now. It does exist on the web somewhere else, so i'll just have to
find it before i can post here with the results.
Since the point of this thread is to do 1000 LEDs however, that will
take a bit more planning and perhaps even dividing up the LEDs
One thing i havent done yet is come up with a general formula for this
new type of multiplexing but it's not hard to understand once
you see the schematic and a very rough description.
I'll start looking and get back here, unless someone else can find
a link before then.
Here is a drawing, from memory, of the traditional way of
Charlieplexing vs true three state multiplexing. As shown,
Charlieplexing only allows one LED to be turned on at a
time, while with true three state multiplexing more than one
LED can be turned on at any given time. For the simple
example shown, turning any two LEDs on at a time means
raising the duty cycle by a factor of 2, which increases average
current by a factor of 2, which is significant. This has
even more of an impact with larger displays, and i hope i
can find that better drawing as the turn on pattern technique
is much more obvious than it looks at first.
Of particular interest is that the duty cycle for Charlieplexing
is 1/12 of the total scan time, while for turning two LEDs on
at any given time the duty cycle for any LED is 1/6, which is
If i remember right, a 100 LED matrix can be done with a duty
cycle of 20 percent per LED, which is MUCH better than only
1 percent per LED with traditional Charlieplexing. This means
that with Charlie the average current through any LED would
be Ipeak/100 which would be roughly 1ma per LED (not a lot!)
but with true three state multiplexing would be 100/5=20ma,
which is very good for most small LEDs. This clearly illustrates
the huge impact this can have on a display.
The disadvantage of course is that more resistors are required,
and a few more control lines might also be needed in some cases
in order to divide the LEDs up properly for correct scanning.
In this drawing, the black L is a constant low, never turned off,
and the red H is a high that turns the red LED on constantly.
The blue H is a high that turns on the blue LED, and the
green L is a low that turns on the green LED. For that line
#3 the choice is to either apply a low or a high depending
on what other LED is to be turned on. Driving line 1 is